1. Field of the Invention
This invention relates generally to nonvolatile memory device structure and operation. More particularly, this invention relates to circuits and method for operation of a NAND like dual charge retaining transistor NOR flash nonvolatile memory device.
2. Description of Related Art
Nonvolatile memory is well known in the art. The different types of nonvolatile memory include Read-Only-Memory (ROM), Electrically Programmable Read Only Memory (EPROM), Electrically Erasable Programmable Read Only Memory (EEPROM), NOR Flash Memory, and NAND Flash Memory. In current applications such as personal digital assistants, cellular telephones, notebook and laptop computers, voice recorders, global positioning systems, etc., the Flash Memory has become one of the more popular types of Nonvolatile Memory. Flash Memory has the combined advantages of the high density, small silicon area, low cost and can be repeatedly programmed and erased with a single low-voltage power supply voltage source.
The Flash Memory structures known in the art employ a charge retaining mechanism such as charge storage or a charge trapping. In a charge storage mechanism, as with a floating gate nonvolatile memory, the charge representing digital data is stored on a floating gate of the device. The stored charge modifies the threshold voltage of the floating gate memory cell to determine the digital data stored in the floating gate nonvolatile memory cell. In a charge trapping mechanism, as in a Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) or Metal-Oxide-Nitride-Oxide-Silicon (MONOS) type cell, the charge is trapped in a charge trapping layer between two insulating layers. The charge trapping layer in the SONOS/MONOS devices has a relatively high dielectric constant (k) such Silicon Nitride (SiNx).
A present day flash nonvolatile memory is divided into two major product categories such as the fast random-access, asynchronous NOR flash nonvolatile memory and the slower serial-access, synchronous NAND flash nonvolatile memory. NOR flash nonvolatile memory as presently designed is the high pin-count memory with multiple external address and data pins along with appropriate control signal pins. One disadvantage of NOR flash nonvolatile memory is as the density is doubled, the number of its required external pin count increases by one due to the adding of one more external address pin to double the address space. In contrast, NAND flash nonvolatile memory has an advantage of having a smaller pin-count than NOR with no address input pins. As density increases, the NAND flash nonvolatile memory pin count is always kept constant. Both main-streamed NAND and NOR flash nonvolatile memory cell structures in production at the present time use one charge retaining (charge storage or charge trapping) transistor memory cell that stores one bit of data as charge or as it commonly referred to as a single-level program cell (SLC). They are respectively referred as one-bit/one transistor NAND cell or NOR cell, storing a single-level programmed data in the cell.
The NAND and NOR flash nonvolatile memories provide the advantage of in-system program and erase capabilities and have a specification for providing at least 100K endurance cycles. In addition, both single-chip NAND and NOR flash nonvolatile memory products can provide giga-byte density because their highly-scalable cell sizes. For instance, presently a one-bit/one transistor NAND cell size is kept at ˜4λ2 (λ being a minimum feature size in a semiconductor process), while NOR cell size is ˜10λ2. Furthermore, in addition to storing data as a single-level program cell having two voltage thresholds (Vt0 and Vt1), both one transistor NAND and NOR flash nonvolatile memory cells are capable of storing at least two bits per cell or two bits/one transistor with four multi-level threshold voltages (Vt0, Vt1, Vt2 and Vt03) in one physical cell. The multi-level threshold voltage programming of the one transistor NAND and NOR flash nonvolatile memory cells is referred to as multiple level programmed cells (MLC).
Currently, the highest-density of a single-chip double polycrystalline silicon gate NAND flash nonvolatile memory chip is 64 Gb. In contrast, a double polycrystalline silicon gate NOR flash nonvolatile memory chip has a density of 2 Gb. The big gap between NAND and NOR flash nonvolatile memory density is a result of the superior scalability of NAND flash nonvolatile memory cell over a NOR flash nonvolatile memory. A NOR flash nonvolatile memory cell requires 5.0V drain-to-source (Vds) to maintain a high-current Channel-Hot-Electron (CHE) injection programming process. Alternately, a NAND flash nonvolatile memory cell requires 0.0V between the drain to source for a low-current Fowler-Nordheim channel tunneling program process. The above results in the one-bit/one transistor NAND flash nonvolatile memory cell size being only one half that of a one-bit/one transistor NOR flash nonvolatile memory cell. This permits a NAND flash nonvolatile memory device to be used in applications that require huge data storage. A NOR flash nonvolatile memory device is extensively used as a program-code storage memory which requires less data storage and requires fast and asynchronous random access.
The act of programming of a Flash nonvolatile memory cell involves charging the charge retaining region (floating gate or charge trapping layer) with electrons which causes the turn-on threshold voltage level of the memory cell to increase. Thus, when programmed, the Flash nonvolatile memory cell will not turn on; that is, it will remain non-conductive, when addressed with a read potential applied to its control gate. Alternately, the act of erasing a Flash nonvolatile memory cell involves removing electrons from the floating gate to lower the threshold voltage level. With the lower threshold voltage level, a Flash nonvolatile memory cell will turn on to a conductive state when addressed with a read potential to the control gate. However, a Flash nonvolatile memory cell suffers from the problem of over-erasure. Over-erasure occurs if, during the erasing step, too many electrons are removed from the floating gate leaving a slight positive charge. This biases the memory cell slightly on, so that a small current may leakage through the memory cell even when it is not addressed.
Currently, as discussed in U.S. Pat. No. 6,407,948 (Chou), the most commonly used Flash memory erasing methods employ the Fowler-Nordheim tunneling phenomena and the channel hot-electron tunneling phenomena. In an erasing operation for a Flash nonvolatile memory cell, a voltage is continually applied to a Flash nonvolatile memory cell to generate a voltage field with a negative potential difference between the control gate and the drain or channel of a Flash nonvolatile memory cell. Electrons accumulated in the floating gate of a Flash nonvolatile memory cell are reduced because the electrons pass through a thin dielectric layer of the Flash nonvolatile memory cell to cause a reduction of the threshold voltage of the Flash memory cell. When the erasing operation is performed, an erasing voltage pulse is applied to each Flash memory cell of a Flash memory array to erase all of the Flash memory cells in the array. However, not all of the Flash memory cells of the Flash memory array have the same circuit characteristics. Some of the Flash memory cells will suffer over-erasure. An over-erased Flash memory cell is one in which a threshold voltage is less than +0.5 volts. When the Flash memory array has multiple over-erased Flash memory cells on multiple columns of the Flash memory cells, the Flash nonvolatile memory cell operates as though it were a depletion device and provides a leakage current. This leakage current causes the data reading accuracy of the Flash memory array to be adversely affected. During a read operation of selected a Flash nonvolatile memory cells, the bit line connected to the selected Flash memory cell is also connected to any over-erased Flash memory cells connected to the bit line. The bit line will suffer from excess leakage current while reading the non-conducting Flash memory cell.